1. Field of the Invention
The present invention relates to nonlinear optical chromophores and, more particularly, pertains to sterically stabilized second-order nonlinear optical chromophores and devices incorporating the same.
2. Description of the Related Art
Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and telecommunications. One of the challenges in this field is to design and synthesize second-order NLO chromophores (the active components of second-order nonlinear optical materials) that simultaneously possess large first molecular hyperpolarizabilities (xcex2), good chemical and thermal stability, and optical transparency at optical communication wavelengths (1.3 and 1.55 xcexcm). Chromophore intermolecular electrostatic interactions prevent the simple scaling of molecular optical nonlinearity into macroscopic optical nonlinearity. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electrooptic activity) by electric field poling or self-assembly methods. Chromophores with xcex2 values many times those of the well-known Disperse Red 19 dye are thus required to obtain electrooptic coefficients comparable to or higher than those of the leading commercial material crystalline lithium niobate.
The value of xcex2 for a chromophore can be increased by using a diene moiety in place of thiophene in the conventional phenylethenylenethiophene xcfx80-conjugated bridge. Moreover, this enhancement in xcex2 can be accomplished without an increase in the wavelength of the charge-transfer absorption xcexmax. However, the resulting phenylpolyene bridge has poor thermal stability unless the polyene structure is sterically protected. The synthesis of various sterically-protected (ring-locked) phenylpolyene chromophores involves cyclic enones such as isophorone, verbenone and double-ring locked dienone as starting materials and intermediates. The Knovenegal coupling reaction between enones and electron acceptors is the critical step in both backward and forward methods reported. The low reactivity of enone severely limits the choice of acceptor to only a few molecules including malononitrile, isoxazolone, and thiobarbituric acid and therefore has become the bottleneck in the development of ring-locked phenylpolyene-bridged high xcex2 chromophores.
A new class of ring-locked aminophenylpolyenal donor-bridges has been developed. These new donor-bridges, according to the present invention, have very high Knovenegal reactivity and have been coupled with a variety of acceptors bearing acidic methyl or methylene groups (including the most desired TCF and TCI type of acceptors shown in FIG. 11) to obtain a new class of second-order NLO chromophores. This methodology broadens the scope of polyene-bridged chromophores without significantly sacrificing thermal stability or optical transparency. This synthetic approach leads to the development of device-quality NLO chromophores (shown in FIG. 1) possessing xcexcxcex2 values (where xcexc is the chromophore dipole moment) of 15,000xc3x9710xe2x88x9248 esu or greater at 1.9 xcexcm as determined by the electric field induced second harmonic generation (EFISH) technique.
A variety of different molecular structures are possible for the chromophores of the present invention. An exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group and a bridge structure therebetween, with the electron acceptor group being double bonded to the bridge structure. In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group and a ring-locked bridge structure between the electron donor group and the electron acceptor group. The bridge structure comprises, for example, two protected alicyclic rings or ring-locked trienone.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, a ring-locked tricyano electron acceptor group, and a bridge structure therebetween. In a preferred embodiment, the electron acceptor group comprises an isophorone structure.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group, and a bridge structure therebetween, with the bridge structure including a bithiophene unit. In a preferred embodiment, the bridge structure further includes a modified isophorone unit.
The NLO materials of the present invention are suitable for a wide range of devices. Functions performed by these devices include, but are not limited to, the following: electrical to optical signal transduction; radio wave to millimeter wave electromagnetic radiation (signal) detection; radio wave to millimeter wave signal generation (broadcasting); optical and millimeter wave beam steering; and signal processing such as analog to digital conversion, ultrafast of signals at nodes of optical networks, and highly precise phase control of optical and millimeter wave signals.